The present invention relates to a semiconductor light-emitting device such as a semiconductor laser using a gallium nitride (GaN) based compound semiconductor, and a method of fabricating the same. More particularly, the invention relates to a semiconductor light-emitting device in which a current blocking layer for ensuring precise current injection only into the desired light-emitting region is formed in close proximity to a light-emitting layer, and a method of fabricating the same.
Techniques have been widespread for fabricating light-emitting devices, such as semiconductor lasers emitting red or infrared laser light by using GaAs or AlGaInP based compound semiconductors. On the other hand, there has been a demand for semiconductor light-emitting devices that would emit blue light of a shorter wavelength; using GaN based compound semiconductors, it has now become possible to produce light-emitting diodes that emit blue light, and blue semiconductor lasers also are currently under development.
As for semiconductor laser structures, gain waveguide structures such as shown in FIGS. 3(a) to 3(c) have been considered for semiconductor lasers. That is, in the structure of FIG. 3(a), the upper (p-side) electrode 25 is patterned and the current injection region is restricted to a narrow region under the electrode pattern; in the structure of FIG. 3(b), protons or the like are bombarded on both sides of the current injection region to form high-resistivity regions defining the current injection region; in the structure of FIG. 3(c), both sides of the current injection region are etched away by dry etching, etc. to produce a mesa structure. In each of FIGS. 3(a) to 3(c), reference numeral 21 is, for example, an n-type GaAs substrate, on top of which an n-type cladding layer 22, an active layer 23, and a p-type cladding layer 24 are formed one above another to form a double-heterojunction structure semiconductor laser; 25 is a p-side electrode, 26 is an n-side electrode, and 27 is a proton bombarded region.
However, with the electrode pattern alone, it is not possible to perfectly define the current injection region, and much current is wasted. On the other hand, with the method of forming high-resistivity regions surrounding the current injection region, not only equipment cost is high but the depth of the high-resistivity regions cannot be controlled with good accuracy. Further, with the method of forming a mesa by etching, since GaN based compound semiconductor layers cannot be etched with good accuracy, the etching depth to form the mesa varies, causing variations in emission characteristics. Moreover, in any of these methods, since the electrode pattern and high-resistivity regions for defining the current injection regions, and the etched portions for forming the mesa, are formed in locations separated from the light-emitting layer, the region that actually leads to the light-emitting layer spreads beyond the current injection region, resulting in much leakage current and thus reducing the emission efficiency.
On the other hand, there is a demand for a semiconductor laser of a refractive index waveguide structure in which a current blocking layer of the opposite conductivity type to that of a light confinement layer is formed within the light confinement layer, with a stripe formed in the current blocking layer to define the current injection region. One method considered for making a semiconductor laser of this structure using GaN based compound semiconductor layers is as follows. That is, as shown in FIG. 4(a), GaN based compound semiconductor layers consisting of an n-type buffer layer 2, an n-type cladding layer 3, an active layer 4, and a p-type cladding layer 5, and a current blocking layer 7 of n-type GaN are epitaxially grown one above another, for example, on top of a sapphire substrate 1 by using MOCVD (metal-organic chemical vapor deposition) equipment. Here, the n-type cladding layer 3, the active layer 4, and the p-type cladding layer 5 together constitute a light-emitting layer forming portion 14. Thereafter, as shown in FIG. 4(b), the current blocking layer 7 is etched to form a stripe recess 17, after which a semiconductor layer is epitaxially grown again by using the MOCVD equipment. Since these semiconductor layers usually contain gaseous nitrogen among their constituent components, they are often grown epitaxially by using MOCVD equipment.
However, as earlier mentioned, the etching of the GaN based compound semiconductor layers cannot be controlled with good accuracy. The etching proceeds not only into the current blocking layer 7 but also into the light-emitting layer forming portion 14 such as the p-type cladding layer 5 and the active layer 4, damaging the light-emitting layer forming portion 14 and degrading the emission characteristics. Conversely, if the etching is insufficient, the current blocking layer 7 of the opposite conductivity type remains unetched, as a result of which the current does not flow and emission cannot be achieved. With the above structure, therefore, semiconductor lasers with stable characteristics cannot be obtained, and no commercial implementations are available yet. Moreover, when the substrate with epitaxially grown layers is taken out the MOCVD equipment for etching or other processing, Ga in the GaN based compound semiconductor layers is oxidized, thus contaminating the surface. The problem is that this degrades the quality of epitaxial growth of GaN based compound semiconductor layers in subsequent processing.
For semiconductor lasers using GaAs based compound semiconductors, on the other hand, there is sometimes employed a method in which after forming a current blocking layer a stripe groove is etched, followed again by epitaxial growth of a semiconductor layer. In GaAs based compound semiconductors also, if Al is contained, oxidation can easily occur when exposed to air; in this case, a method may be employed in which after the etching, thermal etching is performed in an epitaxial growth furnace to evaporate part of the semiconductor layers for cleaning of the surface, and then epitaxial growth is performed. For GaAs based compound semiconductors, since MBE (molecular beam epitaxy) equipment is usually used for epitaxial growth, thermal etching can be performed by evacuating the equipment to a vacuum, so that precise etching only of the current blocking layer of GaAs can be accomplished. For GaN based compound semiconductors, on the other hand, since gaseous nitrogen is contained among their constituent elements, epitaxial growth is often performed by an MOCVD process using MOCVD equipment. However, in the case of MOCVD equipment, the degree of vacuum cannot be raised, and thermal etching cannot be performed as precisely as for GaAs based compound semiconductors. More specifically, a high degree of vacuum would facilitate evaporation of GaN and make etching control possible, but since the degree of vacuum cannot be raised, GaN does not easily evaporate and, hence, etching control is difficult.
In this way, in semiconductor light-emitting devices using GaN based compound semiconductors, since etching cannot be controlled with good accuracy, the current injection region cannot be accurately defined in close proximity to (within a distance of 0.1 to 1 .mu.m from) the light-emitting layer. The resulting problem is that much current is wasted, making it impossible to improve the emission efficiency.
Furthermore, as described above, MOCVD equipment cannot be evacuated to a high degree of vacuum, and the device surface cannot be cleaned in the equipment where semiconductor layers are grown. This leads to the problem that once the semiconductor layer structure is taken out the MOCVD equipment for etching or other processing, a GaN based compound semiconductor layer of high quality cannot be epitaxially grown on top of it in the MOCVD equipment, and as a result, semiconductor light-emitting devices, such as semiconductor lasers having stripe grooves and light-emitting diodes with well-defined light-emitting regions, cannot be obtained.